Energetic electron plasma blanket



June 20, 1967 R. v. NElDlGH ETAL 3,326,769

ENERGETIC ELECTRON PLASMA BLANKET Filed July 20, 1966 wmzam 0.."

INVENTOHS. Rodger VNe/o'igh William L. Stirling ATTORNEY.

United States Patent ENERGETIC ELECTRON PLASMA BLANKET Rodger V. Neidigh, Knoxville, and William L. Stirling,

Oak Ridge, Tenn, assignors to the United States of America as represented by the United States Atomic Energy Commission Filed July 26, 1966, Ser. No. 566,716 Claims. (Cl. 176-4) The invention described herein was made in the course of, or under, a contract with the US. Atomic Energy Commission.

The present invention relates to means for producing an energetic electron blanket symmetrical about the axis of an evacuated chamber provided with magnetic rnirror fields. As described in US. Patent No. 3,160,566, issued August 9, 1962, to R. A. Dandl et al., an energetic plasma blanket is desirable for surrounding any ionized plasma in order to minimize the instreaming of neutral particles thereinto, such that the plasma may attain sufiicient density and temperature to promote or effect the production of substantial numbers of neutrons. In the above patent, the provision of high power, radio frequency radiation caused a plasma to be formed in a magnetic field so as to create an electron plasma blanket on the surface of a central plasma. However, in that patent the blanket was made up largely of high energy electrons and the density and temperature of such electrons were substantially large such that the X-ray production therefrom was large and objectionable and substantial shielding was required to be placed between the device and personnel operating the device for protective purposes.

In the patent application of John W. Flowers et al., Serial No 367,266, filed May 13, 1964, now Patent No. 3,268,758 issued August 23, 1966, there is described means for establishing a hollow cylindrical discharge in -an evacuated enclosure provided with a magnetic field by placing a gas-fed cathode and an anti-cathode off the axis of symmetry of a cylindrical anode and creating a PIG-type discharge between the cathode and anti-cathode, and the discharge forms a cylindrical shape through the anode and symmetrical about the axis of the anode because of the electric fields involved. The hollow discharge of the Flowers et a1. application may be considered a blanket for stopping neutral particles. However, it is not a very effective blanket because the electrons therein are relatively cool; that is, they have a temperatureof about l0-20 ev.

Thus, there exists a need for a means for producing a relatively hot electron blanket while at the same time keeping the density of the high-energy electrons sufiicient- 1y low such as to minimize the production of the objectionable X-rays.

' With a knowledge of the limitations of the prior art as discussed above, it is the object of the present invention to provide a means for creating a hollow, energetic electron blanket Within an evacuated enclosure permeated by a magnetic field for production and containment of energetic plasmas, such that the distribution of the X-ray producing, energetic electrons in the blanket is minimized.

This and other objects and advantages of the present invention will become apparent upon a consideration of the following detailed specification and the accompanying drawing, wherein:

The single figure is a cross-sectional view of a device for producing a hot electron blanket in accordance with the above object.

The above object has been accomplished in the present invention by providing a pressure gradient arc discharge, such as disclosed in the patent of Rodger V. Neidigh, No. 2,928,966 issued March 15, 1960, in an evacuated chambar provided with a magnetic field, with the discharge being produced by electrodes which are mounted parallel with but off-center of the axis of the chamber, such that a very energetic electron discharge extending between a cathode and a gas-fed anti-cathode will cause hot electrons to drift azimuthally out of said discharge about the axis of the chamber to form a hollow, energetic electron blanket which extends the full length of the machine. The Neidigh pressure gradient discharge, as utilized in the present invention, is not only mounted off-axis of the evacuated chamber, but there is also provided gas feed to the anti-cathode which was not provided in the Neidigh patent, and the advantages thereof as well as other advantageous modifications will be discussed hereinafter.

Referring to the drawing, there is shown an apparatus for accomplishing the above object. In the drawing, a vacuum vessel or chamber 1 is surrounded near its end face plates 2 and 3 with magnet coils 4, 5. A vacuum of the order of 3x10 Torr is obtained in the chamber 1 by connecting the chamber to suitable vacuum pumps, not shown, through an opening 23 in a conventional manner. Mounted on face plate 2 is means for producing a pressure gradient arc discharge such as described in the above-mentioned Neidigh Patent No. 2,928,966. However, the arc discharge of the present invention is operated at a relatively high (2025 kv.) cathode potential as compared to 3.5 kv. cathode potential of the Neidigh discharge. This is done to provide higher electron energies and densities for forming a hot electron blanket in a manner to be described below. It should be understood that the discharge of the present invention may be and can be operated at a cathode potential in a range from 3 to 25 kv., for example. However, it is preferred to operate in the higher range, that is, from 2()25 kv., to provide the desired energy and density of electrons for the discharge and resulting blanket. In order to operate the discharge at the higher cathode potential and at higher midplane magnetic fields between the mirror coils 4, 5, it was necessary to provide several modifications to the Neidigh arc discharge producing means which will be discussed hereinafter.

A heated tantalum cathode 10 is spaced about one inch from a hollow, gas-fed anode 11. Means, not shown, are provided for water cooling the anode 11. Gas is fed into the interior of the anode 11 at a controlled rate from a gas source, not shown, by means of a gas feed tube 15. Cathode 10 is supported byan electrically insulated cathode holder 12. A source of high voltage 6 is connected to the cathode 10 by means of leads 7, 8. In order to eliminate the DC. power losses which cause rapid and severe erosion of the anode during operation of the discharge using a high (20-25 kv.) cathode potential, a shield 9 is postioned between the anode 11 and the cathode. This shield 9 is a copper plate which is water cooled by means of cooling tubes 13. Plate 9 is provided with a hole into which'the cathode filament projects. Optimum operating conditions are achieved when the cathode is positioned with its emitting surface flush with that side of the shield facing the anode. The shield 9 is connected to operate at cathode potential. by means of the lead 7.

In the other end of the vacuum chamber 1 there is provided a grounded baffie 17 and an anti-cathode 18 which is electrically floating. In the device of the abovementioned Neidigh patent, the anti-cathode 'Was a flat plate which was adequate for his reflexing discharge since his maximum cathode potential was 3.5 kv. However, in the present invention where a cathode potential from 20 to 25 kv. at 1 ampere is utilized, and percent of the input power is deposited on the anti-cathode, it was necessary to modify the structure of the anti-cathode from that used in the Neidigh device in order to dissipate the high power densities, that is, 30 to 50 kw. per cm. in the present device. Instead of a fiat plate, an iron shield beam expander is provided which includes the anti-cathode 18 and an iron shield 19 which encompasses a recessed, opened well of the anti-cathode, as shown, and water cooling is provided for the iron shield beam expander.

Another modificationover the above Neidigh device is that gas is fed to the anti-cathode and this is accomplished by feeding gas at a controlled rate from a source, not shown, through a gas feed tube 25 into the interior of the recessed portion of the anti-cathode 18. Thus, feed gas, which may be hydrogen, for example, is now fed into the reflex discharge 16 from both the anode, as before, and the anticathode, with between 66 and 90 percent of the total gas input being fed into the anti-cathode, depending on the operating conditions. Feeding gas into the anti-cathode has the desirable effect of increasing the electron density and improving the operational stability of the discharge and the resultant blanket.

The means for water cooling the above iron shield beam expander includes cooling water inlets 28 and 29 from a source of cooling water, not shown, and the water is directed through suitable passageways, as shown, between the iron shield 19 and the anti-cathode 18, and is finally directed to water outlet tubes 30 and 31. It should be understood that the gas feed tube 25, the water inlet tubes 28, 29, and the water outlet tubes 30 and 31 may all be enclosed in a common header unit extending through the face plate 3, if such is desired.

The vacuum chamber 1 is provided with an inner, cylindrical liner 26 defining an inner chamber 20 and provided with open ends through which one or more reflex discharges are adapted to pass, and liner 26 is also provided with one or more openings 24 to facilitate the evacuation of chamber 20. Liner 26 is supported by means of a plurality of brackets 27. Also, the outside of the liner 26 is provided with cooling tubes 21 such that the liner may be cooled as with liquid nitrogen, if desired. The cooled liner serves as an effective cold trap for pump-oil vapors, etc. The use of such cooling efiects an increase in the electron density in the blanket 22 by a factor of 2 to 3.

The spacing between the cathode 10 and the anticathode 18 is about 5 feet, for example, The axis of the arc unit 10, 11 and the anti-cathode unit 18, 19 are mounted about 6 inches from the magnetic axis of the device, for example. The energetic electrons produced by the pressure gradient are are trapped between the magnetic mirrors provided by the coils 4, 5 and,'being off the axis, experience a strong radial magnetic field gradient. Thus, they precess azimuthally to form a plasma cylinder or blanket 22. Due to the magnetic field shape, this produces a blanket that conforms to the field lines, and the blanket is about 22 cm. in radius at the midplane of the chamber 20, with the midplane thickness of the hollow plasma being 35 cm.

Some of the operating parameters of the device of the present invention, as described above, are as follows. The vacuum in chamber 20 is maintained at about 3X10 Torr, and the pressure within the anode 11 is maintained at about Torr, as in the above-mentioned Neidigh patent, to thus provide for a pressure gradient discharge 16 in a manner similar to that patent. The shield 9 and the cathode 10 are connected to potential of 20-25 kv., the anode 11 and the bafile 17 are grounded, and the anti-cathode 18 is electrically floating. The feed gas to the anode 11 and to the anti-cathode 18 may be hydrogen, for example, with the greater portion of the total gas feed being fed to the anti-cathode, as mentioned above. The hollow anode 11 is 6 inches long, for example, and has an inside diameter of a selected value from /8 inch to inch. The use of an anode with the y -inch inner diameter is preferred because it effects a large increase in the electron density in the discharge as compared to the use of the /s-inch-inner-diameter anode in the device, particularly when two arcs are utilized as discussed below.

In order to permit operation of the device of the present invention at a higher midplane magnetic field, which is desirable for containing energetic ions inside the blanket 22, a magnetic field shaping electromanget coil 14 is provided which encompasses the hollow anode 11, as shown. Without the use of the electromagnet 14, the axial, midplane field could not be raised above about 1500 gauss using the above cathode potential of 20-25 kv., since raising the field higher has the equivalent effect of throwing a short across the arc electrodes 10, 11 causing the discharge to extinguish. The mirror coils 4, 5 have a mirror ratio of 2:1, such that the magnetic field in the mirror regions at the anode 11 and cathode 10 end or at the anti-cathode end has a maximum permissible value of about 3000 gauss without the use of the coil 14. However, by providing the field shaping coil 14, it is now possible to operate the device at substantially higher midplane magnetic field strengths without raising the magnetic field in the anode 11 above the maximum permissible value of 3000 gauss. The coil 14 is capable of substantially bucking out an applied magnetic field up to about 18 kilogauss in the anode mirror region, such that the midplane magnetic field strength can be adjusted to such a value as to effect the optimum heating of the electrons in the hollow electron blanket 22, which optimum value is not possible without the use of the coil 14. The electrons are continuously heated by an electron beam-plasma interaction in the blanket during the above optimum heating conditions therein. The coil 14 is water cooled by means of a cooling tube 32, for example.

It should be understood that other means can be used to reduce the magnetic field in the anode channel other than the field shaping coil as discussed above. For example, an iron cylinder can be placed around the anode in order to reduce the field in the anode channel, such a cylinder being about one inch shorter than the anode, for example. Thus, a stronger magnetic field in the device would be necessary to bring the field value in the anode channel back up to the original operating value. For example, using the iron cylinder about the anode, the optimum midplane magnetic field that can be achieved is about 3100 gauss and the field in the center of the anode is about 3000 gauss. This is about the same value of field in the anode when operating at 1500 gauss axial midplane field with no iron shielding about the anode. However, the power input to the device is limited with the use of the iron cylinder due to the iron end effect and saturation of the iron such that attempts to achieve a power input in the vicinity of 40 kw. resulted in destruction of the anode. Thus, the use of the field shaping coil is preferred over the use of the iron cylinder since higher input power can be used with the former which does not have the objectionable iron end effect at the higher input powers. It should be understood that the above-described device can be, and in some instancse preferably, operated without the field shaping means when a less dense electron plasma blanket is desired and such operation is in the overall scope of the present invention.

It should be noted that the device described above is not limited to the use of a single arc. Multiple arcs can be utilized, and a second arc has been operated in conjunction with the first arc. The electrodes of the second are are displaced radially 6 inches from the magnetic axis, like the first arc, for. example, with the two arcs being positioned azimuthally apart. The two arcs may be operated simultaneously or separately. Only the cathode holder 12', high power lead 8', and the cathode 10' of the means for producing a pressure gradient, second arc are shown in the drawing. The corresponding other components of the second are producing means are not shown on the drawing for the sake of clarity, such other components including those identical to the components 6, 7, 9, 11, 13, 14, 15, 17, 18, 19, 25, 2'8, 29, 30, 31, and 32 of the first are producing means. The cathode holder 12 and cathode filament are shown displaced'slightly from their actual position for the sake of clarity. It should be understood that the cathode 10' is placed in axial alignment with the edge of the electronblanket 22 opposite to the edge of the blanket along the discharge 16. It should be noted that the use of anodes with a /s-inch inner diameter produced no gain in the hot electron density in the blanket when double arc operation was utilized instead of single arc operation at low power input (7 kv.). However, when the -inch-inner diameter anodes were utilized, double arc operation produced a substantial increase (about double) in the hot electron density as compared to single arc operation, with the power input being 23 kv. in both cases. It should be understood that single arc operation produces a sufficiently hot electron blanket for the purposes intended, that is, for confining an ionized plasma within the interior of the blanket, for ionizing the background gas for building up the plasma density and temperature of the plasma confined inside of the blanket, and for protecting the ionized plasma confined by the blanket from the instreaming of neutral particles from outside the blanket, with the blanket acting as a means for ionizing such neutral particlesa's they come in contact with the blanket. In some instances, double arc operation may be preferred. However, single arc operation is more attractive'economically in that'less feed gas is required than for double arc operation and the power supply requirements are, of course, less than for double arc operation.

In the operation of the above-described device, at least three electron distributions have been observed in the blanket: (l) a high energy, X-ray producing component, mentioned above, of density 10 to 10 cm? and temperature about 100 kev.; (2) an intermediate energy component of density between 1 and 5 10 cm.* and temperature about 2 kev.; and (3) a low energy component of density greater than 10 cm? and temperature of a few (about 2.5) electron volts. The temperature and density of the electron plasma can be measured by a combination of four different methods: (1) X-ray energy distribution and total energy emitted, (2) Langmuir probe, (3) power probe, and (4) microwave cut-off experiment. A NaI scintillation spectrometer can be used for detection of the X-rays. The 2-kev. electrons are concentrated in the approximately 5-cm.-thick, visible annulus center of the blanket, and the l00-kev. electrons are distributed both radially inward and outward from the central region. Experiments in which a thin tantalum padde is projected into the blanket annulus show that the maximum X-ray intensity is achieved when the paddle is about 1 inch from the visible plasma glow. The X-ray production is stopped when the paddle interrupts even a part of the glowing region on midplane of the chamber 20. Similar behavior is observed when probing radially outward from within the blanket. X-ray intensity is maximum just inside the glow, and production is again stopped upon intercepting the glow. The intermediate low energy electron distributions have been investigated only with the probes and microwaves, since no X-rays are detected on the scintillation spectrometer when a paddle experiment removes the hot or high energy electron distribution, but the background gas is still being ionized by the 2-kev. electrons as evidenced by the streaming of cold plasma out the mirrors. The low energy (cold) electron distribution is provided by the background plasma, while the intermediate energy and high energy electron distributions in the electron blanket are provided by the pressure gradient arc discharge, as described above.

In summary, the means for producing an energetic electron blanket using either a single are or two arcs, as described above, and the use of the magnetic field shaping coil 14 for shaping the field in the anode region thus permitting an optimum electron heating interaction in the electron blanket produced in the operation of the device,

provides an effective means for shielding a plasma confined within the interior of the hollow blanket from the instreaming of neutral particles. Such an energetic blanket will essentially eliminate charge-exchange losses of the plasma within the inside of the blanket, will provide a background for dissociation, ionization, etc., and for trapping, thus effecting an increase in the density and temperature of the plasma confined by the blanket to such a value as to produce a substantial number of neutrons. Also, the energetic electron blanket of the above-described device produces substantially less X-rays than the device of the above-mentioned Dandl et al. Patent No. 3,160,566, and in that patent the gas feed rate is a critical parameter in the operation of the device, while in the device of the present invention the gas feed rate is not critical, but is or myabe varied only to optimize the electron density in the blanket.

This invention has been described by way .of illustration rather than by way of limitation and it should be apparent that it is equally applicable in fields other than those described.

I. A device for producinga hot electron plasma blanket comprising an elongated chamber; means for providing a selected magnetic mirror field in said chamber; means connected to said chamber for evacuation thereof to a selected pressure; an electron emissive filament cathode and a hollow, water-cooled, gas-fed anode closely spaced therefrom, said anode having an inside diameter of a selected value from to inch, said anode and cathode being mounted in one end of said chamber and positioned radially off-axis a preselected distance. from the axis of said chamber; a gas-fed anti-cathode mounted offaxis in the other end of said chamber and provided with a beam expander recessed portion, said recessed portion of said anti-cathode being in alignment with said anode and cathode; a flat-cooled, centrally apertured shield encompassing said filament cathode with the face of said shield being positioned flush with the emitting surface of said cathode; an iron shield member encompassing said recessed portion of said anti-cathode; means for cooling said anti-cathode and iron shield member; a source of high voltage supply connected to said filament cathode and to said apertured shield; a source of feed gas; means for feeding said feed gas at controlled rates to the hollow portion of said anode and to the recessed portion of said anti-cathode with the major portion of the total feed gas being fed to said anti-cathode, said rate of gas feed to said anode being such as to maintain a higher pressure within said anode than the pressure within said chamber, whereby a pressure gradient, reflexing, off-center are discharge is established between said filament cathode and said anti-cathode through said hollow anode, said olfcenter are discharge producing a rotationally drifting hot electron plasma about the axis of said chamber to form a substantially cylindrical, energetic electron plasma blanket along the magnetic field lines provided by said mirror field, the electrons in said blanket being continuously heated by an electron beam-plasma interaction.

2. The device set forth in claim 1, wherein said chamber is provided with a longitudinal inner liner with open ends for passage of said discharge and blanket therethrough, said liner defining an inner chamber for substantially enclosing said blanket, said liner being provided with at least one opening such as to maintain the vacuum pressure within said inner chamber at essentially the same selected value that exists exterior to said inner chamber.

3. The device set forth in claim 2, wherein said inner liner is provided with cooling tubes mounted on the exterior thereof, a source of cooling fluid, and means for feeding said cooling fluid through said liner cooling tubes.

4. The device set forth in claim 3, wherein said means for providing said magnetic mirror field includes a pair of electromagnetic coils encompassing said cathode and anode and a pair of electromagnetic coils encompassing said anti-cathode, said coils having a mirror ratio of 2:1; said selected magnetic field being about 3000 gauss in the vicinity of said coils and about 1500 gauss at the midplane between said pairs of coils; said source of high voltage supply being of a selected value in the range from 20 to 25 kv.; and said feed gas being hydrogen.

5. The device set forth in claim 4, wherein the various recited means for producing said arc discharge are duplicated to produce an identical, second arc discharge, said second arc discharge being positioned radially off-axis from the axis of said chamber the same distance as said first arc discharge is displaced from said chamber axis, said two are discharges being positioned azimuthally 180 apart, the anodes of said are discharge producing means having an inside diameter of inch, wherein the electron density. in said blanket is at least doubled over that achievable with single arc operation.

6. The device set forth in claim 3, wherein said means for providing said magnetic mirror field includes a pair of electromagnetic coils encompassing said chamber about said cathode and anode and a pair of electromagnetic coils encompassing said chamber about said anti-cathode, said coils having a mirror ratio of 2:1; means concentrically disposed about said hollow anode for shaping the magnetic field in the anode channel region, whereby said magnetic mirror field can be increased by a factor of at least two over that permissible without the field shaping means to maintain said energetic blanket stable during high voltage operation of said device.

7. The device set forth in claim 6, wherein said magnetic field shaping means comprises a water-cooled electromagnetic coil connected in such a manner as to oppose the magnetic field provided 'by said pair of coils 8 encompassing said chamber about said anode and cathode.

8. The device set forth in claim 6, wherein said magnetic field shaping means comprises an iron cylinder.

9. The device set forth in claim 7, wherein said selected magnetic fieldis about 3100 gauss in the midplane of said device between said pairs of coils, with a corresponding value of about 6200 gauss in the vicinity of said pairs of coils, said field shaping means reducing the magnetic field in the said anode channel region to a value of about 3000 gauss; said source of high voltage supply being of a selected value-in the range from 2025 kv.; and-said feed gas being hydrogen.

10. Thedevice set forth in claim 9, wherein the various recited means for producing said are discharge are duplicated to produce an identical, second arc discharge, said second arc discharge being positioned radially off-axis from the axis of said chamber the same distance as said first arc discharge is displaced from said chamber axis, said two arc discharges being positioned-azimuthally apart, the anodes of said are discharge producing means having an inside diameter of inch, wherein the electron density in said blanket is atleast doubled over that achievable with single arc operation.

References Cited UNITED STATES PATENTS 2,928,966 3/1960 Neidigh 313-63 3,160,566 12/1964 Dandl et al. 1767 3,268,758 8/1966 Flowers 3l3161 X REUBEN EPSTEIN, Primary Examiner. 

1. A DEVICE FOR PRODUCING A HOT ELECTRON PLASMA BLANKET COMPRISING AN ELONGATED CHAMBER; MEANS FOR PROVIDING A SELECTED MIRROR FIELD IN SAID CHAMBER; MEANS CONNECTED TO SAID CHAMBER FOR EVACUATION THEREOF TO A SELECTED PRESSURE; AN ELECTRON EMISSIVE FILAMENT CATHODE AND A HOLLOW, WATER-COOLED, GAS-FED ANODE CLOSELY SPACED THEREFROM, SAID ANODE HAVING AN INSIDE DIAMETER OF A SELECTED VALUE FROM 5/8 TO 5/16 INCH, SAID ANODE AND CATHODE BEING MOUNTED IN ONE END OF SAID CHAMBER AND POSITIONED RADIALLY OFF-AXIS A PRESELECTED DISTANCE FROM THE AXIS OF SAID CHAMBER; A GAS-FED ANTI-CATHODE MOUNTED OFFAXIS IN THE OTHER END OF SAID CHAMBER AND PROVIDED WITH A BEAM EXPANDER RECESSED PORTION, SAID RECESSED PORTION OF SAID ANTI-CATHODE BEING IN ALIGNMENT WITH SAID ANODE AND CATHODE; A FLAT-COATED, CENTRALLY APERTURED SHIELD ENCOMPASSING SAID FILAMENT CATHODE WITH THE FACE OF SAID SHIELD BEING POSITIONED FLUSH WITH THE EMITTING SURFACE OF SAID CATHODE; AN IRON SHIELD MEMBER ENCOMPASSING SAID RECESSED PORTION OF SAID ANTI-CATHODE; MEANS FOR COOLING SAID ANTI-CATHODE AND IRON SHIELD MEMBER; A SOURCE OF HIGH VOLTAGE SUPPLY CONNECTED TO SAID FILAMENT CATHODE AND TO SAID APERTURED SHIELD; A SOURCE OF FEED GAS; MEANS FOR FEEDING SAID FEED GAS AT CONTROLLED RATES TO THE HOLLOW PORTION OF SAID ANODE AND TO THE RECESSED PORTION OF SAID ANTI-CATHODE WITH THE MAJOR PORTION OF THE TOTAL FEED GAS BEING FED TO SAID ANTI-CATHODE, SAID RATE OF GAS FEED TO SAID ANODE BEING SUCH AS TO MAINTAIN A HIGHER PRESSURE WITHIN SAID ANODE THAN THE PRESSURE WITHIN SAID CHAMBER, WHEREBY A PRESSURE GRADIENT, REFLEXING, OFF-CENTER ARC DISCHARGE IS ESTABLISHED BETWEEN SAID FILAMENT CATHODE AND SAID ANTI-CATHODE THROUGH SAID HOLLOW ANODE, SAID OFFCENTER ARC DISCHARGE PRODUCING A ROTATIONALLY DRIFTING HOT ELECTRON PLASMA ABOUT THE AXIS OF SAID CHAMBER OF FORM A SUBSTANTIALLY CYLINDRICAL, ENERGETIC ELECTRON PLASMA BLANKET ALONG THE MAGNETIC FIELD LINES PROVIDED BY SAID MIRROR FIELD, THE ELECTRONS IN SAID BLANKET BEING CONTINUOUSLY HEATED BY AN ELECTRON BEAM-PLASMA INTERACTION. 